The invention relates to a hybrid filter for an AC mains system, with the hybrid filter having a passive and an active filter which are electrically connected in series, and with the active filter having a voltage source.
Such a hybrid filter, but used for a DC mains system, is known from the article xe2x80x9cApplication of Power Active Filters for Mitigation of Harmonicsxe2x80x9d, printed in the Proceedings of xe2x80x9cStockholm Powertechxe2x80x9d, Stockholm, June 1995. The active filter has a voltage source and generates voltages in order to eliminate harmonics in the mains system. For this purpose, this active filter is not connected directly to the mains system, but is electrically connected in series with a coupling circuit, which limits the current and voltage requirements for the active filter. This coupling circuit may be a transformer, a coupling capacitor, a tuned filter or a combination of these elements. In the Proceedings article, the active filter is linked to the DC mains system by means of a tuned filter and a transformer. Such a filter arrangement is also referred to as a hybrid filter.
A double filter is provided as the tuned filter. A double filter results in low impedances at a number of resonant frequencies and with a fundamental having a relatively low wattless component. Only the capacitor bank need be designed for high voltage, thus justifying the considerable cost advantage of a double filter.
The passive filter in an AC mains system is often also used for power-factor correction. The fundamental component of the filter current is thus relatively high, as is the case, for example, in a high-voltage DC transmission system (HVDC system). If an active filter is now connected in series with such a passive filter, this results in a high load on the active filter since the fundamental component also flows through the active filter. Such a load may result in the active filter being disconnected from the hybrid filter after a short time, in order to protect the semiconductor components in its pulsed-current converter. However, the semiconductor components may also be designed for the fundamental component of the filter current. This strategy has the disadvantages that, firstly, there are probably no commercially available semiconductor components which on the one hand have the desired current and voltage load capacity and on the other hand have a high switching frequency, and, secondly, that the active filter, and hence the hybrid filter, is very expensive, so that the cost-benefit ratio of such a hybrid filter is no longer economic.
The article xe2x80x9cEin Regelverfahren zur aktiven Filterung in der Starkstromtechnikxe2x80x9d [A control method for active filtering for high-power technology] by C. Filtermann, F. Hillenbrand and C. Landgraf, printed in the German Journal xe2x80x9cRegelungstechnikxe2x80x9d [Control engineering], 1982, Issue 8, pages 263 to 270, proposes a concept on the basis of which the voltage source of the active filter is no longer loaded by the fundamental current. This concept is illustrated schematically in FIG. 2a of this article and comprises a parallel tuned circuit, which is electrically connected in parallel with the hybrid filter and is tuned to the mains system frequency. In consequence, the voltage source of the active filter is no longer loaded with the fundamental current, so that it only need be matched to the harmonics. However, additional measures are required for any power-factor correction that is necessary.
The invention is now based on developing such a known hybrid filter in such a way that the fundamental component of the filter current is kept remote from its active filter in a simple manner, while allowing power-factor correction to be carried out by its passive filter.
Since a filter which is tuned to the mains system frequency of the AC mains system is electrically connected in parallel with the active filter of the hybrid filter, the fundamental component of the filter current is dissipated to the reference potential. For this reason, this filter is also referred to as a dissipation path. This dissipation path represents approximately a short-circuit for the fundamental component of the filter current while, in contrast, it has a significant resistance in the active-filter frequency band. Since the fundamental component of the filter current of the hybrid filter flows through the dissipation path, the active filter is no longer loaded by this fundamental component, so that the active filter need be designed only for the harmonic currents which are to be eliminated. A hybrid filter which is known for a DC mains system can thus also be used for an AC mains system, by simple means, without any need to overdesign the active filter.
A tuned filter allows a very low impedance to be achieved at a resonant frequency, even if the fundamental wattless component of the filter circuit is relatively low. However, the filter can easily be mistuned if there is a mains-system frequency discrepancy or the capacitor capacitance changes due to temperature. This mistuning results in a considerable proportion of the fundamental component of the filter current flowing through the active filter. This load on the active filter can result in the active filter being disconnected from the hybrid filter. The hybrid filter can thus no longer carry out its task, thus making the cost-benefit ratio even worse.
In one particularly advantageous embodiment of this hybrid filter, a regulator arrangement is provided, is connected on the input side to two current transformers and is connected on the output side by means of an adder to a control input of the active filter, with one of the two current transformers being associated with the active filter, and with the other being associated with the passive filter. This regulator arrangement makes it possible to remove the proportion of the fundamental component of the filter current flowing through the active filter as a result of mistuning. This considerably reduces, or eliminates, the load on the active filter due to fundamental current components of the filter current during a transient process and during continuous operation, despite mistuning of the dissipation path.
Advantageous refinements of the hybrid filter according to the invention can be found in the dependent claims 3 to 5.